Unit: Chemical Quantities Determining the Empirical Formula from Percent Composition Day 6 - Notes

After today, you will be able to… Calculate the empirical formulas for various compounds given their percent makeup

Empirical Formula Determination from % Composition To determine empirical formulas from % composition follow these steps: 1.Assume 100.0g of the compound (change % into grams) 2.Convert g  moles for each element using molar mass 3.Divide each mole answer by the smallest mole answer 4.If you do not have a whole number, multiply by the smallest whole # to get a whole # ratio. 5.Skip to this step if you already have a whole number ratio from #3. These whole #s are the subscripts for each element.

To sum up these steps here is a poem for my favorite little chemists: “Percent to mass Mass to Moles Divide by small Multiply by whole!”

Example: What is the empirical formula for a compound that is 21.2% N, 6.10% H, 24.3% S, and 48.4%O? 21.2gN 6.10gH 24.3gS 48.4gO 1 mol N 14.01gN = N 2 H 8 SO 4 x mol H 1.01gH x 6.04 = 1 mol S 32.07gS = x x 1 mol O 16.00gO 3.03 = = = = = ~ 8 1 4

Example: What is the empirical formula for a compound that is 69.8% Fe and 30.2% O? 69.8gFe 30.92gO 1 mol Fe 55.85gFe = Fe 2 O 3 x mol O 16.00gO 1.89 = x 1.25 = = (2) = = 2 3

Time to Practice! Complete WS 6 HOMEWORK: